Labour MP Tom Watson has said police must investigate claims of a "powerful paedophile ring" linked to a previous prime minister's "senior adviser" and Parliament.

He asked David Cameron to ensure that officers looked into the allegations.

Mr Cameron said he would look into the issue, although he said he was not clear which former prime minister's adviser was being referred to.

The exchanges came during Prime Minister's Questions in the Commons.

Mr Watson said an evidence file collected by the police to convict paedophile Peter Righton in 1992 "contains clear intelligence of a widespread paedophile ring".

"One of its members boasts of his links to a senior aide of a former prime minister, who says he could smuggle indecent images of children from abroad," he said.

"The leads were not followed up, but if the files still exist, I want to ensure that the Metropolitan Police secure the evidence, re-examine it and investigate clear intelligence suggesting a powerful paedophile network linked to Parliament and No 10."

David Cameron said it was a "very difficult and complex case" and he was "not entirely sure" which former prime minister Mr Watson was referring to.

But he agreed to look at the case "very carefully and see what the government can do" to give Mr Watson the assurances he wanted.